Here's the spectrum culture review for those that can't be arsed clicking on Denno's link:

The Mars Volta founder Omar Rodríguez-López considers Octahedron the band's "acoustic album." Though the explosive, off-beat prog rhythms, distorted guitars and electronic textures may render this release a far cry from the genuine acoustic-based simplicity of Nirvana's MTV Unplugged in New York or something even a little more layered like Beck's Mutations, there may be some semblance of backward plausibility to this claim. In comparison to the rest of The Mars Volta's sonically charged, climax-packed work, Octahedron is one of the most blatant stylistic facelifts since Metallica nearly went alt-rock with Loaded. Like their metal predecessor, The Mars Volta attempts to fine-tune a hybrid between their characteristic sound and a new path. Octahedron is generously peppered with haunting acoustic passages. On tracks like "Since We've Been Wrong," these passages gradually build into a full-on emotional rock assault. For reference, think of the subdued track "Televators" from De-Loused In The Comatorium. "With Twilight As My Guide" and "Copernicus" come as close to purebred acoustic numbers as The Mars Volta have ever offered, with this latter song perhaps being their most minimalist work to date. If this is their new direction, then fans have nothing to worry about. On "Twilight," the band harnesses their creativity into some of their most compelling atmospheric interplays. The Volta's rarely-seen simplistic side of songwriting really stands out; it's almost refreshing to take a breather with these uncharacteristically accessible numbers.

The band also transposes this straightforward approach to many harder tracks. "Desperate Graves" sparingly interrupts its steady, pop-laden, four-chord approach with the band's signature progressive explosions, while on "Halo of Nembutals," they're too busy riding on a steep wave of emotional excess to generate the chaotic, offbeat energy of their previous compositions. This is not to suggest Octahedron is completely devoid of the Volta's outlandishly experimental ways. "Cotopaxi" is a Zeppelin riff gone mad; Tom Pridgen's drums flutter around time signatures faster than the listener can breathe, while the rest of the ensemble immerses itself in its gloriously bitter and demented shell of blues.

But is it enough to satisfy fans hungry for more dense fare? That depends. Those willing to unconditionally embrace a more traditional form of The Mars Volta will probably enjoy this album. The prog-purists, however, will have a harder time swallowing a more linear meal, especially with several instrumental freakouts confined to superfluous codas. To the average listener, Octahedron is yet another entertaining, if slightly more subdued, dose of aural Armageddon. To expect another rock milestone would be too demanding. With musicianship this apt, Octahedron still knocks most of its contemporaries out of the water.